1. Field of the Invention
The present invention is in the field of current-to-frequency converters, and specifically discloses a circuit for converting very small bipolar currents ranging, for example, from 10.sup.-14 to 5.times.10.sup.-6 A. to frequencies ranging from 0 to 5 MHz. The currents to be measured typically originate in mass spectrometers, electron capture detectors, flame-ionization detectors, photomultipliers, etc., and the output of the circuit is typically used in digital circuits.
2. The Prior Art
A technique for generating pulses of precisely controlled charge content is disclosed in U.S. Pat. No. 3,921,012 to Marshall III.
The technique used by Marshall bears a resemblance to one disclosed in U.S. Pat. No. 3,022,469 to Bahrs et al.; in U.S. Pat. No. 3,289,271 to Gray; and in U.S. Pat. No. 3,742,389 to Henrickson.
As taught by those patents, the technique consists of applying a voltage increment to one plate of a capacitor. Both the voltage increment and the capacitance are precisely controlled. In response to the applied voltage increment, a quantity of charge Q=C .DELTA.V flows into the opposing capacitor plate. The key advantage of this feedback pulse generation technique is that the quantity of charge Q in each pulse is independent of the voltage waveform used to drive the capacitor. In addition to achieving potentially greater accuracy, this capacitive technique results in a wider dynamic range and elimination of the stray capacitance problems associated with other methods.
In the Gray and Hendrickson patents referred to above, the charge feedback pulses are additively combined with the current to be measured without attenuation. This approach is adequate where the input currents to be converted are relatively large, but smaller charge feedback pulses must be generated or obtained to give the instrument greater sensitivity if minute input currents are to be converted to frequency with satisfactory accuracy.
In the Marshall III invention, the succession of charge pulses generated are applied to a conductor connecting a plate of each of two capacitors which function as a charge division circuit for the applied charge. The charge, having no direct current outlet path, builds up voltage on the capacitors, which would render the circuit inoperable in the relatively short time it takes to charge to the limit of the linear voltage range of the charging transistor. It is therefore necessary periodically to provide a dead time in the system operation during which the accumulated charge is cleared away by a specially provided discharge circuit. The need for a dead time places sampling period limitations on the circuit and thereby makes the conversion of current to frequency discontinuous.
In certain situations, such as when fast sampling is required, pulses lost during dead-times may produce significant errors and therefore constitute a fundamental limitation to minimum sampling period.
In discussing the prior art, Marshall, III, states that "current to frequency conversion using charge feedback through semiconductors cannot achieve the low leakage currents and high impedances necessary to perform the direct digitization of currents as small as 10.sup.-14 A. without the complexity of constant external zero correction." As will be seen below, Applicant has invented a charge dividing circuit employing semiconductors which does exactly that.
The Marshall patent provides the prior art which is most nearly relevant to the present invention. A search that was conducted found the following patents that were addressed to similar problems but were far less relevant:
In U.S. Pat. No. 3,778,794 Szabo, et al, disclose an analog-to-pulse rate converter in which the input signal to be measured charges an integrating capacitor. A charge-level threshold is provided such that when the charge level on the integrator capacitor exceeds the threshold, a discharge control signal is initiated by control logic to remove equal predetermined amounts of capacitor charge from the integrator capacitor during successive clock cycles. The amount of charge in each of the discharging pulses is determined by applying a constant current for a fixed portion of the clock pulse period. This is a different technique and less precise than that used in the present invention. The charge delivered is the integral of the current waveform, and it is difficult to maintain precise control of the waveform at high frequencies and over a wide range of frequencies.
A current-to-frequency converter similar to that disclosed by Szabo, et al., is described in "High-Speed Charge-to-Current Data Domain Converter for Analytical Measurement Systems" by Woodruff and Malmstadt in Analytical Chemistry, Vol. 46, No. 9, August, 1974, Page 1162. The precisely determined charge feedback pulses are generated by using a high speed analog switch to alternately connect and disconnect a very stable reference voltage (V) across a resistor of resistance (R). This provides a stable current I = V/R. Since the switch is driven by a very accurate crystal clock oscillator, each ON time .DELTA.t is accurately determined and the charge content is each pulse q = .DELTA.t is also precisely determined.
The limitations of the mode of generating the feedback charge pulses used in the invention of Szabo, et al., and described in the paper by Woodruff and Malmstadt are described in Columns 2-4 of Marshall III (referred to above), which Columns are incorporated herein by reference. These problems include inability to precisely control the width of the pulse driving the resistor as well as inability to control the amplitude of the voltage pulse particularly at the megahertz repetition rates employed. Other problems include limited dynamic range and the undesirable effects of stray capacitance.
A well known class of analog-to-frequency instruments bears a superficial similarity with the present invention. Although those instruments use integrators and threshold devices comparable to those used in the present invention, the principle on which they operate is different.
In those prior-art instruments, a current or voltage to be measured is integrated over time by an integrator. The output of the integrator is applied to a threshold device which emits a signal when the threshold is exceeded. This threshold signal is used to initiate resetting of the integrator to its initial condition. The signal generated by the threshold device is not added to the quantity being measured so as to reduce or null that quantity. Instead, the signal generated by the threshold device always resets the output of the integrator to its initial condition.
Whatever accuracy the instruments may have depends on how precisely the threshold level and the integrating capacitance can be controlled. This is in contrast to the operating principle of the present invention, wherein a sequence of predetermined charge packets is additively combined with the input quantity to nullify it. In the present invention, the accuracy is independent of the threshold level and depends mainly on the precision with which the feedback charge pulses can be generated and applied.
The following patents are believed to fall into the latter-described class of instruments: U.S. Pat. No. 3,942,110 to Milkovic; U.S. Pat. No. 3,902,139 to Harrell; U.S. Pat. No. 3,660,782 to Friedman, et al.; U.S. Pat. No. 3,376,431 to Merrell; and U.S. Pat. No. 3,594,649 to Rauch. This class of instrument is also described in a paper by Lucero, Smith and Johnson, "A Hydrogen Flame Ionization Detector for Martian/Lunar Life Detection Experiments" published in Instrumentation in the Aerospace Industry, Volume 16, Page 176-186, (1970) by the Instrument Society of America (Paper No. LC 69-59 467).